Occurrence of the disease known as cataracts, in which the lens of the eye becomes clouded, is common, and can lead to blindness. It has become accepted practice to alleviate this condition by surgically removing the cataract-effected lens and replacing it by an artificial intraocular lens.
The cataract-effected lens is usually removed by manual extraction or phaco-emulsification. Manual extraction requires expression of the nucleus of the lens through a wound of about 12 mm in length.
The technique known as phaco-emulsification, as described, for example, in U.S. Pat. No. 3,589,363, enables removal of a cataract-effected lens through a much smaller incision of about 2.5-4 mm, for example, 3.2 mm. This is accomplished using high frequency ultrasound energy, typically of 40 kHz frequency, that is transmitted by a phaco-emulsification needle to fragment or emulsify the nucleus of the cataract-effected lens. Once fragmented or emulsified, the nuclear material is aspirated through a lumen of the phaco-emulsification needle.
During aspiration of the fragmented nucleus, a simultaneous flow of liquid into the eye is provided around the needle via a soft plastic or elastomeric sleeve concentrically disposed over the needle to form an annulus. This flow of liquid into the eye is essential to prevent collapse of the anterior chamber of the eye while the fragmented or emulsified nucleus is aspirated via the phaco-emulsification needle. Also, the inflowing liquid serves to cool the needle, thus reducing heat generated by the ultrasonic vibration of the needle. If this heat were instead permitted to be transmitted to the entry wound of the eye, thermal damage of the cornea or scleral tissue could result.
One difficulty encountered in practicing the phaco-emulsification technique is the necessity of maintaining a low-leakage seal between the entry wound in the eye and the sleeve surrounding the phaco-emulsification needle. If the wound entrance is too small, the sleeve may be compressed against the needle, thereby restricting the flow of irrigant into the anterior chamber and allowing frictional heat to be transmitted to the wound.
Alternatively, if the wound is made large to avoid compression of the sleeve, unacceptably high leakage may occur around the sleeve, for example, at rates of 25 cc/minute. The liquid lost by leakage must be replaced by liquid inflow through the annulus between the plastic sleeve and the needle, thus reducing the safety margin in maintaining a constant anterior chamber volume. Accordingly, to avoid collapse of the anterior chamber, liquid inflow through the annulus should never be less than total liquid outflow via the phaco-emulsification lumen and leakage around the sleeve.
In response to these concerns, a number of techniques have been developed which attempt to reduce leakage from the wound by improving the seal between the wound and the sleeve, while avoiding heat transmission to the wound.
A first technique uses a rigid plastic sleeve made from a material such as polysulphone rather than a softer silicone material, because polysulphone resists compression by the wound. Another technique, described in U.S. Pat. No. 5,282,786, incorporates a rigid plastic or metal sleeve disposed on the outer circumference of a softer silicon sleeve, so that the rigid plastic contacts the entry wound. These techniques, however, have the drawback that they increase wound distortion and tend to enlarge the wound during the procedure, thus enhancing leakage.
A second technique, described for example in U.S. Pat. Nos. 5,286,256 and 5,354,265, employs a rigid plastic or metal intermediate sleeve inserted in the annulus between the conventional outer soft silicone sleeve and the phaco-emulsification needle. The rigid intermediate sleeve is permitted to "float" on the exterior of the phaco-emulsification needle, and the needle may have a reduced outer diameter in a middle region to limit longitudinal travel of the intermediate sleeve. The reduced middle region of the needle permits a small incision while maintaining fluid flow around the phaco-emulsification needle. However, such benefits are believed to be more than offset by the increase in the overall diameter of the apparatus to accommodate the intermediate sleeve and the overall reduction in the irrigation cross-sectional flow area caused by the presence of the intermediate sleeve.
A yet third technique attempting to solve the problems of fluid leakage from the entry wound and heat transmission is described in U.S. Pat. Nos. 4,634,420, 4,643,717, 4,808,154 and 5,242,385. The sleeves described in these patents each include a plurality of inwardly extending ribs that serve to reinforce the sleeve against compression, to limit contact between the sleeve and the phaco-emulsification needle, and to provide flow channels in the event that the sleeve is compressed against the needle. All of these designs share the common drawback that the ribs enhance the rigidity of the sleeve, and thus pose a risk of transmitting more frictional heat to the entry wound. In addition, all of these designs require a larger entry wound to accommodate the added dimension of the ribs. Moreover, if the ribs are formed of silicone or other soft plastic or elastomeric material, it is expected that the compressive loads transmitted to such sleeves from the entry wound may compress the ribs to an extent that unacceptably low flow rates might still occur.
In view of the foregoing, it would be desirable to provide a phaco-emulsification needle that overcomes the drawbacks of previously known needle and sleeve arrangements by permitting the use of a small wound size, and that reduces the risk of restriction of fluid flow around the needle.
It further would be desirable to provide a phaco-emulsification needle that provides reduced contact area between the needle and the sleeve, thus reducing the transmission of heat to adjacent tissue, with concomitant reduction in the risk of tissue damage.
It would be still further desirable to provide a phaco-emulsification needle that ensures adequate irrigant flow even in those cases where the sleeve is compressed against the exterior of the needle.
It would be yet further desirable to provide a device for use with a phaco-emulsification needle that would improve fluid flow and reduce heat transmission in those cases where the sleeve is subjected to high compressive loading by the entry wound.